It is known to print images, especially variable images, on pre-printed stock. For example, U.S. Pat. No. 3,858,777 (Rodek/Xerox Corporation) describes an apparatus for printing prerecorded information on a pre-printed web. The image carried on the pre-printed web may be, for example business forms, it being necessary to print the variable data in registration with the layout of the forms. The web carries uniformly spaced marks which are detected by a sensor as the web is unwound from a roll. By detection of the marks, the displacement of the web can be determined. Variable information to be printed on the web, is carried on a film strip. The speed of the film strip through an exposure device can be adjusted to match the determined speed of the web.
The printer described by Rodek operates on the basis of the substrate web moving at a nominally constant speed and of the provision of uniformly spaced marks on the web. In practice, the requirements may be difficult to meet accurately. Variations in operating conditions or in the condition of the web, may result in minor variations in the web speed. Furthermore, after the substrate web has been pre-printed, it is rolled and stored prior to use, during which some dimensional changes in the web, such as stretching and relaxing, may occur. The result of this is that marks pre-printed on the web may no longer be equally spaced. Furthermore, it may well be desirable to pre-print images of different lengths, with the result that the use of uniformly spaced marks on the web is no longer appropriate. The printer described by Rodek also has the disadvantage that, should any change in web speed be made, a correction in the timing of the exposure is made at a point which may be located in the middle of an image. An image carrying such a intermediate correction is undesirable.
U.S. Pat. No. 5,315,323 (Ohshima et al./Ricoh Company Ltd.) describes a color image forming apparatus for forming an image on a paper web by sensing a register mark formed on the paper by a writing head. The distance between the register mark and the sensor and the distance between the writing head and the paper web are kept constant. This arrangement is not suitable for the printing of pre-printed stock.
European patent application EP 624477 (Canon K K) describes a method in which an image including a mark is printed on a substrate by screen printing. Another image is then printed on the substrate by ink-jet printing on the basis of the detection of the pre-printed mark. The mark detector is mounted on the ink jet printing head. The substrate is fed past the ink jet printing head in a step-wise manner, the mark detector being moved laterally into a parking position as printing continues. This arrangement does not allow for the printing of an image to be initiated at a position other than at the location of the mark. Furthermore, the use of step-wise transport of the substrate is to be avoided for high quality high speed print work.